Computed tomography (CT) is a medical imaging method or modality employing tomography, i.e., imaging by sections or sectioning, created by computer processing. Digital geometry processing can be used to generate a three-dimensional image of the inside of an object from a series of two-dimensional X-ray images taken around a single axis of rotation. CT data can be manipulated to demonstrate various bodily structures based on their ability to block an X-ray beam.
Magnetic Resonance Imaging (MRI) can provide more contrast between different soft tissues than CT, making it especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging. MRI employs radio frequency (RF) fields to alter the static magnet induced magnetic alignment of the subject nuclei, for example hydrogen atoms, in the subject to produce a rotating magnetic field. This field can be detected and used to produce images of the subject.
Positron emission tomography (PET) is a nuclear medicine imaging technique or modality, which can produce a three-dimensional image of functional processes in the body, for example the functioning of an organ. In PET, a radioactive tracer radioisotope is introduced into a subject, typically by injection. The positron emitting radioisotope occurs at a higher concentration in regions of high cellular metabolic activity. When an emitted positron encounters a free electron, the positron and electron may annihilate into two gamma photons which inherently provides higher signal to noise ratio than single photon emission imaging. These gamma photons can be detected by scintillation crystals, i.e., a material that emits light upon absorbing the gamma photons. The light emitted from the scintillation crystal can then be converted to electrical charge by an electronic light sensor, such as a photomultiplier tube (PMT) or avalanche photodiode (APD). The light sensor converts the light emitted by the scintillation crystal into a time varying stream of charge, i.e. an exponentially decaying current with decay time representative of the scintillation crystal. The resulting current produces a measurable electrical pulse; either current or impedance converted voltage may be used to measure the resulting total charge originating in the light sensor. Based on the time coincidence of the electrical pulses and the total energy measurements, three-dimensional images of the measured concentration of the tracer in the subject's body can be produced.
It can be beneficial to combine different modalities. For example, it can be beneficial to combine a CT scanner and a PET scanner in order to provide information about the functioning of an organ and information about the anatomical structures surrounding the organ. A scanner combining a CT modality and a PET modality can be referred to as a multi-modality scanner or a PET-CT scanner. A problem exists in multi-modality scanners, because multi-modality scanners require the signal architecture to accommodate concurrent operation of the modalities. Accommodation of concurrent operation of the modalities can result in less than optimized signal architectures for each individual modality.
The signal processing electronics used with the photo detectors in a commercial PET-CT scanner use vacuum tube based photomultiplier tubes (PMTs) to convert the light from individual scintillation crystals into electrical signals. However, for the development of a combined MRI-PET scanner, the photomultiplier tubes need to be replaced with another type of photo detector insensitive to the time varying electromagnetic fields of an MRI system. One candidate photodetector, which is MRI compatible, is an Avalanche photodiode (APD) biased in the linear range. The APD photo detector does not have the large signal gain achievable using photomultiplier tubes; typically PMTs are operated with anode gains on the order of 106. Commercially available APDs used in PET detectors and are typically operated linearly with a much lower gain in the range of about 100 to 200.
A need exists, therefore, for a data processing system for multi-modality imaging. It would be desirable to provide a system, method and/or apparatus to optimize the signal gain from an Avalanche photodiode in an multi-modality MRI-PET scanner.